Steps to reproduce bug
1. Create new score for C Trumpet
2. Add a C note to the staff
3. Right-click on an empty part of the staff, choose Staff Properties
4. Change instrument to Bb Trumpet
5. Press "OK"

Expected behavior: The note added in step 2 should be written as a D (so that it still sounds the same pitch during playback). The key signature should be D Major.

Actual behavior: The note added in step 2 remains written as an untransposed C (during playback it sounds a major second lower). The key signature is still C Major.

Workaround: Press "Concert pitch" before step 3 and the score you get after step 5 works as expected.

MuseScore version: r.2878 nightly

(Operating System: Windows 7)